Additive particles for improving optical performance of an electrophoretic display

ABSTRACT

The present invention is directed to an electrophoretic fluid which comprises additive particles. The concentration of the additive particles in the electrophoretic fluid is 2.5% to 25% by weight and the additive particles are not seen at the viewing side during operation of the display. The resulting fluid can improve optical performance of a display device, such as image stability and contrast ratio. The present invention is also directed to an electrophoretic display comprising the electrophoretic fluid.

This application is a continuation-in-part of U.S. application Ser. No.14/535,172, filed Nov. 6, 2014; which is a continuation-in-part of U.S.application Ser. No. 13/243,751, filed Sep. 23, 2011, now U.S. Pat. No.8,902,491, This application is also a continuation-in-part of U.S.application Ser. No. 14/179,458, filed Feb. 12, 2014; which claimspriority to U.S. Provisional Application No. 61/765,550, filed Feb. 15,2013. The above-identified applications are incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

The present invention is directed to an electrophoretic fluid, inparticular, an electrophoretic fluid comprising additive particles forimproving optical performance of an electrophoretic display.

BACKGROUND OF THE INVENTION

An electrophoretic display (EPD) is a non-emissive device based on theelectrophoresis phenomenon influencing charged pigment particlessuspended in a dielectric solvent. An EPD typically comprises a pair ofspaced-apart plate-like electrodes. At least one of the electrodeplates, typically on the viewing side, is transparent. Anelectrophoretic fluid composed of a dielectric solvent with chargedpigment particles dispersed therein is enclosed between the twoelectrode plates.

An electrophoretic fluid may have one type of charged pigment particlesdispersed in a solvent or solvent mixture of a contrasting color. Inthis case, when a voltage difference is imposed between the twoelectrode plates, the pigment particles migrate by attraction to theplate of polarity opposite that of the pigment particles. Thus, thecolor showing at the transparent plate can be either the color of thesolvent or the color of the pigment particles. Reversal of platepolarity will cause the particles to migrate to the opposite plate,thereby reversing the color.

Alternatively, an electrophoretic fluid may have two types of pigmentparticles of contrasting colors and carrying opposite charges and thetwo types of pigment particles are dispersed in a clear solvent orsolvent mixture. In this case, when a voltage difference is imposedbetween the two electrode plates, the two types of pigment particleswould move to opposite ends. Thus one of the colors of the two types ofthe pigment particles would be seen at the viewing side.

Further alternatively, multiple types of charged pigment particles maybe present in an electrophoretic fluid for forming a highlight or fullcolor display device.

In an ideal fluid, the charged pigment particles remain separate and donot agglomerate or stick to each other or to the electrodes, under alloperating conditions. In addition, all components in the fluid must bechemically stable and compatible with other materials present in anelectrophoretic display.

For all types of the electrophoretic displays, the fluid containedwithin the individual display cells of the display is undoubtedly one ofthe most crucial parts of the device. The composition of the fluiddetermines, to a large extent, the lifetime, contrast ratio, switchingrate and bistability of the device.

Prior to the present invention, it was proposed that the image stabilityof an electrophoretic display may be improved by adding a polymeradditive into an electrophoretic fluid. The polymer additive is eithercompletely dissolved in the fluid or partially dissolved in the fluid toform micelle aggregates. However the usefulness of this approach islimited because adding a polymer additive to the fluid would inevitablyincrease the viscosity of the fluid, resulting in an increase of theswitching time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an electrophoretic fluid comprising one type of chargedpigment particles and also additive particles, both dispersed in asolvent or solvent mixture.

FIGS. 2a-2d show how the uncharged or lightly charged neutral buoyancyparticles may improve the performance of an electrophoretic fluid whichcomprises two types of charged pigment particles.

SUMMARY OF THE INVENTION

The present invention is directed to an electrophoretic fluid comprisingcharged pigment particles and additive particles, all of which aredispersed in a solvent or solvent mixture. The additive particles may beopaque or transparent.

In a first aspect of the present invention, the additive particles areuncharged or slightly charged neutral buoyancy particles. In oneembodiment, the fluid comprises two types of charged pigment particlesof contrasting colors and carrying opposite charge polarities. In oneembodiment, the two types of charged pigment particles are black andwhite, respectively. In one embodiment, the uncharged or lightly chargedneutral buoyancy particles have the same color as one of the two typesof charged pigment particles. In one embodiment, the uncharged orlightly charged neutral buoyancy particles have a color different fromeither one of the two types of charged pigment particles. In oneembodiment, the fluid comprises only one type of charged pigmentparticles.

In one embodiment, the lightly charged neutral buoyancy particles carrya charge which is less than 50%, preferably less than 25% and morepreferably less than 10%, of the average charge carried by thepositively or negatively charged pigment particles.

In one embodiment, uncharged or lightly charged neutral buoyancyparticles are formed from a material selected from the group consistingof polyacrylate, polymethacrylate, polystyrene, polyaniline,polypyrrole, polyphenol and polysiloxane. In one embodiment, theuncharged or lightly charged neutral buoyancy particles are formed froma material selected from the group consisting of poly(pentabromophenylmethacrylate), poly(2-vinylnapthalene), poly(naphthyl methacrylate),poly(alpha-methystyrene), poly(N-benzyl methacrylamide) and poly(benzylmethacrylate).

In one embodiment, the uncharged or lightly charged neutral buoyancyparticles are formed from a material having a refractive index differentfrom that of the solvent or solvent mixture. In one embodiment, theuncharged or lightly charged neutral buoyancy particles are formed froma material having a refractive index higher than that of the solvent orsolvent mixture.

In one embodiment, the uncharged or lightly charged neutral buoyancyparticles are core-shell particles. In one embodiment, the core particleis formed from an organic or inorganic pigment. In one embodiment, theshell is formed from a material selected from the group consisting ofpolyacrylate, polymethacrylate, polystyrene, polyaniline, polypyrrole,polyphenol and polysiloxane. In one embodiment, the shell is formed froma material selected from the group consisting of poly(pentabromophenylmethacrylate), poly(2-vinylnapthalene), poly(naphthyl methacrylate),poly(alpha-methystyrene), poly(N-benzyl methacrylamide) and poly(benzylmethacrylate).

In one embodiment, the size of the uncharged or lightly charged neutralbuoyancy particles is in the range of about 100 nanometers to about 5microns.

In one embodiment, the concentration of the uncharged or lightly chargedneutral buoyancy particles in an electrophoretic fluid is more than 2.5%by weight, but not exceeding about 25% by weight. In one embodiment, theconcentration of the uncharged or lightly charged neutral buoyancyparticles in an electrophoretic fluid is in a range between about 3% toabout 15% by weight. In one embodiment, the concentration of theuncharged or lightly charged neutral buoyancy particles in anelectrophoretic fluid is in a range between about 3% to about 10% byweight.

In a second aspect of the present invention, the additive particles aretransparent particles. In one embodiment, the charged pigment particlesare non-transparent. In one embodiment, the refractive index of thetransparent particles is substantially the same as that of the solventin which they are dispersed. In one embodiment, the refractive index islower than 1.5. In one embodiment, the transparent particles take upless than 20% by volume of the fluid. In another embodiment, thetransparent particles take up less than 10% by volume of the fluid. Inone embodiment, the transparent particles are formed from an organicmaterial. In another embodiment, the transparent particles are formedfrom an inorganic material. In one embodiment, the transparent particlesare formed from a monomer or oligomer selected from the group consistingof acrylate or methacrylate, siloxane modified acrylate or methacrylateand halogenated acrylate or methacrylate. In one embodiment, thetransparent particles have an average size of less than 0.5 μm, or lessthan 0.3 μm or less than 0.1 μm.

In one embodiment, the non-transparent charged pigment particles arewhite particles which carry a positive or negative charge polarity. Inanother embodiment, the non-transparent charged pigment particles areblack and white particles carrying opposite charge polarities.

In one embodiment, the transparent particles are non-charged. In anotherembodiment, the transparent particles are charged. In a furtherembodiment, the transparent particles carry a charge the polarity ofwhich is the same as that carried by one type of the non-transparentcharged pigment particles, but have a different level of mobility thanthat of the non-transparent charged pigment particles.

In the present invention, the solvent in an electrophoretic fluid may bea hydrocarbon solvent. Alternatively, the solvent may be halogenated orfluorinated. The fluid may further comprise a charge control agent.

In a third aspect of the present invention, an electrophoretic displaycomprises display cells which are filled with an electrophoretic fluidas described above. In one embodiment, the display cells are cup-likemicrocells. In another embodiment, the display cells are microcapsules.

In addition, the additive particles in an electrophoretic fluid of thepresent invention are not seen at the viewing side where the images areviewed.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have discovered that by including additiveparticles in an electrophoretic fluid, a display device can have notonly improved image stability but also improved contrast ratio, withoutsignificantly affecting the switching speed.

The charge intensity referred to in the application may be measured interms of zeta potential. In one embodiment, the zeta potential isdetermined by Colloidal Dynamics AcoustoSizer IIM with a CSPU-100 signalprocessing unit, ESA EN# Attn flow through cell (K:127). The instrumentconstants, such as density of the solvent used in the sample, dielectricconstant of the solvent, speed of sound in the solvent, viscosity of thesolvent, all of which at the testing temperature (25° C.) are enteredbefore testing. Pigment samples are dispersed in the solvent (which isusually a hydrocarbon fluid having less than 12 carbon atoms), anddiluted to between 5-10% by weight. The sample also contains a chargecontrol agent (Solsperse 17000®, available from Lubrizol Corporation, aBerkshire Hathaway company; “Solsperse” is a Registered Trade Mark),with a weight ratio of 1:10 of the charge control agent to theparticles. The mass of the diluted sample is determined and the sampleis then loaded into the flow through cell for determination of the zetapotential.

The term “lightly charged” is defined as having a charge which is lessthan 50%, preferably less than 25% and more preferably less than 10%, ofthe average charge carried by the positively charged pigment particlesor negatively charged pigment particles.

In a first aspect of the present invention, the additive particles areuncharged or slightly charged neutral buoyancy particles.

The term “neutral buoyancy” refers to particles which do not rise orfall with gravity. In other words, the particles would float in thefluid between the two electrode plates. In one embodiment, the densityof the neutral buoyancy particles may be the same as the density of thesolvent or solvent mixture in which they are dispersed.

The movement of the particles in an electrophoretic fluid which areuncharged or slightly charged and of neutral buoyancy is not influencedby the electric field generated by the electrode plates. In addition,because of the low concentration (see in a section below), the color ofthe uncharged or slightly charged neutral buoyancy particles is not seenat viewing side of a display device. In other words, only the color(s)of the charged pigment particles in the fluid is/are visible at theviewing side of the display device.

An electrophoretic fluid comprises one type of charged pigment particles(11) and additive particles (which may be un-charged neutral buoyancyparticles) (13), both dispersed in a dielectric solvent or solventmixture (12), as shown in FIG. 1 (i.e., “one particle system”). Thecharged pigment particles (11) would move to be at or near one of theelectrodes (14 a or 14 b), depending on the voltage potential applied tothe electrodes and the charge polarity carried by the charged pigmentparticles (11). One of the two electrodes is on the viewing side whichis where the images are viewed.

The charged pigment particles (11) may be formed from an inorganicpigment, such as TiO₂, ZrO₂, ZnO, Al₂O₃, CI pigment black 26 or 28 orthe like (e.g., manganese ferrite black spinel or copper chromite blackspinel). They also may be formed from an organic pigment such asphthalocyanine blue, phthalocyanine green, diarylide yellow, diarylideAAOT yellow, and quinacridone, azo, rhodamine, perylene pigment seriesfrom Sun Chemical, Hansa yellow G particles from Kanto Chemical, andCarbon Lampblack from Fisher.

The charged pigment particles may also be particles coated with apolymer layer on their surface and the polymer coating can be preparedthrough various conventionally known polymerization techniques.

The charged pigment particles may carry a natural charge or are chargedthrough the presence of a charge controlling agent.

The uncharged or lightly charged neutral buoyancy particles (13) may beformed from a polymeric material. The polymeric material may be acopolymer or a homopolymer.

Examples of the polymeric material for the uncharged or lightly chargedneutral buoyancy particles may include, but are not limited to,polyacrylate, polymethacrylate, polystyrene, polyaniline, polypyrrole,polyphenol, polysiloxane or the like. More specific examples of thepolymeric material may include, but are not limited to,poly(pentabromophenyl methacrylate), poly(2-vinylnapthalene),poly(naphthyl methacrylate), poly(alpha-methystyrene), poly(N-benzylmethacrylamide) or poly(benzyl methacrylate). These materials aresuitable for the neutral buoyancy particles in a system having one, twoor more types of the charged pigment particles.

More preferably, the uncharged or lightly charged neutral buoyancyparticles are formed from a polymer which is not soluble in thedielectric solvent of the display fluid, and also has a high refractiveindex. In one embodiment, the refractive index of the uncharged orlightly charged neutral buoyancy particles is different from that of thesolvent or solvent mixture in which the particles are dispersed.However, typically the refractive index of the uncharged or lightlycharged neutral buoyancy particles is higher than that of the solvent orsolvent mixture. In some cases, the refractive index of the uncharged orlightly charged neutral buoyancy particles may be above 1.45.

In one embodiment, the materials for the uncharged or lightly chargedneutral buoyancy particles may comprise an aromatic moiety.

The uncharged or lightly charged neutral buoyancy particles may beprepared from monomers through polymerization techniques, such assuspension polymerization, dispersion polymerization, seedpolymerization, soap-free polymerization, emulsion polymerization orphysical method, including inverse emulsification-evaporation process.The monomers are polymerized in the presence of a dispersant. Thepresence of the dispersant allows the polymer particles to be formed ina desired size range and the dispersant may also form a layer physicallyor chemically bonded to the surface of the polymer particles to preventthe particles from agglomeration.

The dispersants preferably has a long chain (of at least eight atoms),which may stabilize the polymer particles in a hydrocarbon solvent. Suchdispersants may be an acrylate-terminated or vinyl-terminatedmacromolecule, which are suitable because the acrylate or vinyl groupcan co-polymerize with the monomer in the reaction medium.

One specific example of the dispersant is acrylate terminatedpolysiloxane (Gelest, MCR-M17, MCR-M22),

Another type of suitable dispersants is polyethylene macromonomers, asshown below:

CH₃—[—CH₂—]_(n)—CH₂O—C(═O)—C(CH₃)═CH₂

The backbone of the macromonomer may be a polyethylene chain and n maybe 30-200. The synthesis of this type of macromonomers may be found inSeigou Kawaguchi et al, Designed Monomers and Polymers, 2000, 3, 263.

If the fluid system is fluorinated, the dispersants are then preferablyalso fluorinated.

Alternatively, the uncharged or lightly charged neutral buoyancyparticles may also be formed from a core particle coated with apolymeric shell and the shell may be formed, for example, from any ofthe polymeric material identified above.

The core particle may be of an inorganic pigment such as TiO₂, ZrO₂,ZnO, Al₂O₃, CI pigment black 26 or 28 or the like (e.g., manganeseferrite black spinel or copper chromite black spinel), or an organicpigment such as phthalocyanine blue, phthalocyanine green, diarylideyellow, diarylide AAOT yellow, and quinacridone, azo, rhodamine,perylene pigment series from Sun Chemical, Hansa yellow G particles fromKanto Chemical, and Carbon Lampblack from Fisher or the like.

In the case of core-shell uncharged or lightly charged neutral buoyancyparticles, they may be formed by a microencapsulation method, such ascoacervation, interfacial polycondensation, interfacial cross-linking,in-suit polymerization or matrix polymerization.

The size of the uncharged or lightly charged neutral buoyancy particlesis preferably in the range of about 100 nanometers to about 5 microns.

The uncharged or lightly charged neutral buoyancy particles may beopaque.

In one embodiment as shown in FIG. 2, an electrophoretic fluid maycomprise two types of charged pigment particles (21 a and 21 b) anduncharged or lightly charged neutral buoyancy particles (23), alldispersed in a solvent or solvent mixture (22) (i.e., “two particlesystem”). The movement of the charged pigment particles is determined bythe voltage potential applied to the electrodes (24 a and 24 b).

The two types of charged pigment particles have different opticalcharacteristics. For example, they have contrasting colors and carryopposite charge polarities.

In one embodiment, an electrophoretic fluid may comprise more than twotypes of charged pigment particles.

Each of the types (two or more) of the charged pigment particles mayhave the same characteristics discussed above for the charged particlesin the one particle system.

The suitable materials for the uncharged or lightly charged neutralbuoyancy particles in a system having two or more types of chargedpigment particles may be the same as those described above for the oneparticle system.

In an electrophoretic fluid comprising two types of charged pigmentparticles carrying opposite charge polarities and are of contrastingcolor, the particles preferably have a polymer layer on their surface toprevent them from sticking to each other. Otherwise, in the case of ablack/white display device, the reflectance at the white and blackstates may suffer.

In one embodiment of this aspect of the present invention, the unchargedor lightly charged neutral buoyancy particles (23) added to the fluidmay have a color substantially the same visually as the color of one ofthe two types of charged pigment particles. For example, in a displayfluid, there may be charged black particles, charged white particles anduncharged or lightly charged neutral buoyancy particles and theuncharged or lightly charged neutral buoyancy particles may be eitherwhite or black.

In another embodiment, the uncharged or lightly charged neutral buoyancyparticles may have a color substantially different from the color ofeither one of the two types of charged pigment particles.

FIGS. 2a & 2 b show how the contrast ratio may be improved by theaddition of the uncharged or lightly charged neutral buoyancy particlesin a two particle system. As shown, the presence of the uncharged orlightly charged neutral buoyancy particles, especially if they areformed from a reflective material, increases reflection of the incidentlight (25); thus improving the contrast ratio.

FIGS. 2c & 2 d show how the image stability may be improved by theaddition of the uncharged or lightly charged neutral buoyancy particlesin a two particle fluid system. The un-charged neutral buoyancyparticles can fill in the gaps resulted from the charged pigmentparticles being over packed on the surface of an electrode under adriving electrical field, thus preventing the charged pigment particlesfrom settling due to the gravitational force.

In addition, if the uncharged or lightly charged neutral buoyancyparticles are white, they may enhance the reflectivity of the display.If they are black, they may enhance the blackness of the display.

In a further embodiment of the present invention, the concentration ofthe uncharged or lightly charged neutral buoyancy particles in anelectrophoretic fluid is preferably more than 2.5% by weight, but notexceeding about 25% by weight. In another embodiment, the concentrationof the uncharged or lightly charged neutral buoyancy particles ispreferably in a range between about 3% to about 15% by weight and morepreferably in a range between about 3% to about 10% by weight. The colorof the uncharged or lightly charged neutral buoyancy particles is notseen at the viewing side during operation of the display device.

In a second aspect of the present invention, the additive particles (13)in FIG. 1 are transparent particles which may be charged or uncharged.The transparent particles are usually colorless. They are also not seenat the viewing side during operation of the display device.

In one embodiment, the transparent particles have a refractive indexpreferably lower than 1.5, more preferably to be about 1.4. Therefractive index of the transparent particles is substantially the sameas the refractive index of the solvent in the electrophoretic fluid, sothat the transparent particles do not scatter light and are transparentor close to be transparent in the fluid. The term “substantially thesame” refers to the difference between the two refractive indices notexceeding 10%.

The amount of the transparent particles in an electrophoretic fluid ispreferably less than 20% and more preferably less than 10%, by volume.

The transparent particles may be formed of an organic material, such asa polymeric material. In this case, the starting monomers or oligomersmay be acrylate or methacrylate, siloxane modified acrylate ormethacrylate, halogenated acrylate or methacrylate or monomers that canform a polyurethane.

The monomers or oligomers undergo emulsion polymerization, seedpolymerization, soap-free polymerization, dispersion polymerization,suspension polymerization, phase inversion polymerization or the like,to form the transparent particles.

Examples of the resulting material from polymerization may include, butare not limited to, poly(methyl methacrylate), poly(butyl acrylate),poly(perfluorobutylethyl acrylate), poly(perfluorohexyl ethylmethacrylate) and poly(methacrylate terminated dimethylsiloxanes).

In the polymerization process, a dispersant is preferably present. Thedispersant allows the transparent particles to be formed in a desiredaverage size range (e.g., less than 0.5 μm, preferably less than 0.3 μmand more preferably less than 0.1 μm). The dispersant may also causeformation of a layer physically or chemically bonded to the surface ofthe transparent particles to prevent the particles from agglomeration inthe electrophoretic fluid.

The term “dispersant”, in the context of the present application,broadly includes any materials which promote dispersion or to maintaindispersed particles in a suspension state. Dispersants particularlysuitable for the purpose of the present invention preferably have a longchain (of at least eight carbon atoms or Si—O repeating units) andtherefore they can stabilize the transparent particles in a solvent inthe polymerization process or in the final fluid. Such dispersants maybe an acrylate-terminated or vinyl-terminated macromolecule. They aresuitable because the acrylate or vinyl group can co-polymerize with themonomers or oligomers in the polymerization process.

One specific example of the dispersant is acrylate terminatedpolysiloxane (Gelest, MCR-M17, MCR-M22), as shown below:

The molecular weight of the polysiloxane of the above formula is higherthan 5000.

Another specific example is polyethylene macromonomer of the followingformula:

CH₃[—CH₂—]_(n)—CH₂O—C(═O)—C(CH₃)═CH₂

As discussed above, the backbone of this macromonomer may be apolyethylene chain and n may be 30-200. The synthesis of this type ofmacromonomers may be found in Seigou Kawaguchi et al, Designed Monomersand Polymers, 2000, 3, 263.

If the fluid system is fluorinated or halogenated, the dispersants arethen preferably also fluorinated or halogenated.

In another embodiment, the transparent particles can be made from aninorganic material, such as silica, with a refractive index lower than1.5.

The transparent particles of the present invention may be added to anelectrophoretic fluid comprising one type, two types or multiple typesof charged pigment particles dispersed in a solvent or solvent mixture,as an additive. In a one particle system, one type of charged pigmentparticles is dispersed in a solvent or solvent mixture. In a twoparticle system, two types of pigment particles of contrasting colorsand carrying opposite charge polarities are dispersed in a solvent orsolvent mixture. In a multiple particle system, there may be more thantwo types of pigment particles of different colors and the multipletypes of particles may have different charge polarities, differentlevels of charge intensity or different levels of mobility. The chargedpigment particles referred to in the one particle system, the twoparticle system or the multiple particle system, are non-transparentparticles.

The transparent particles are useful as an additive in anelectrophoretic fluid. For example, when the transparent particles arenon-charged, they can reduce agglomeration between the chargedparticles, thus also reducing the ghosting phenomenon during driving;but they do not have a negative impact on the color exhibition. When thetransparent particles are non-charged, they show no mobility under anelectric field.

If the transparent particles have the same level of charge intensity as,or a higher level of charge intensity than, the charge intensity of thecharged non-transparent particles, they can compete with thenon-transparent particles to prevent the non-transparent particles fromsticking to a dielectric layer.

When the transparent particles are charged, they may carry a charge thepolarity of which is the same as that carried by one type of the chargedpigment particles, and in this case, the transparent particles have adifferent level of mobility than that of the other particles in thefluid.

Example

In a three-neck reaction flask, 200 ml of solvent (silicone oil, DMS-T01from Gelest) is added, followed by adding 32 g of a stabilizer (MCR-M22,Gelest) to the solvent. To the resulting mixture, 16 g of a monomer(methyl methacrylate) is added. Nitrogen is then purged into the flaskand the temperature is increased to 65° C. while stirring. An initiator,LPO (lauryl peroxide), in the amount of about 0.4 g, is added into theflask. The reaction continues for 15 hours, after which polymerparticles are formed. The polymer particles can be separated from theliquid through centrifugation to remove un-reacted species, andre-dispersed into a solvent (Isopar G).

A further aspect of the present invention is directed to anelectrophoretic display wherein display cells are filled with any of thedisplay fluids as described in the present application.

The term “display cell” refers to a micro-container filled with adisplay fluid. A display cell may be a cup-like microcells as describedin U.S. Pat. No. 6,930,818, the content of which is incorporated hereinby reference in its entirety.

A display cell may also be any other micro-containers (e.g.,microcapsules or microchannels), regardless of their shapes or sizes.All of these are within the scope of the present application, as long asthe micro-containers are filled with a display fluid.

The solvent or solvent mixture (12) in which the charged pigmentparticles and additive particles are dispersed preferably has a lowviscosity and a dielectric constant in the range of about 2 to about 30,preferably about 2 to about 15 for high particle mobility. Examples ofsuitable dielectric solvent include hydrocarbons such as isopar,decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils,paraffin oil; silicon fluids; aromatic hydrocarbons such as toluene,xylene, phenylxylylethane, dodecylbenzene and alkylnaphthalene;halogenated solvents such as perfluorodecalin, perfluorotoluene,perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluoro-benzene, dichlorononane, pentachlorobenzene;and perfluorinated solvents such as FC-43, FC-70 and FC-5060 from 3MCompany, St. Paul Minn., low molecular weight halogen containingpolymers such as poly(perfluoropropylene oxide) from TCI America,Portland, Oreg., poly(chlorotrifluoro-ethylene) such as Halocarbon Oilsfrom Halocarbon Product Corp., River Edge, N.J., perfluoropolyalkylethersuch as Galden from Ausimont or Krytox Oils and Greases K-Fluid Seriesfrom DuPont, Delaware, polydimethylsiloxane based silicone oil fromDow-corning (DC-200). The solvent or solvent mixture may be colored by adye or pigment.

More specifically, the solvent in the electrophoretic fluid may be ahydrocarbon solvent, such as dodecane, tetradecane, the aliphatichydrocarbons in the Isopar® series (Exxon, Houston, Tex.) or the like.The solvent can also be a mixture of a hydrocarbon and a halogenatedcarbon or silicone oil base material.

An electrophoretic fluid of the present invention may further comprise acharge control agent, which may be polymeric, non-polymeric, ionic ornon-ionic. The charge control agent may be an ionic surfactant, such assodium dodecylbenzenesulfonate, metal soap, polybutene succinimide,maleic anhydride copolymers, vinylpyridine copolymers, vinylpyrrolidonecopolymer, (meth)acrylic acid copolymers or N,N-dimethylaminoethyl(meth)acrylate copolymers), Alcolec LV30 (soy lecithin), Petrostep B100(petroleum sulfonate) or B70 (barium sulfonate), Solsperse 17000 (activepolymeric dispersant), Solsperse 9000 (active polymeric dispersant),OLOA 11000 (succinimide ashless dispersant), OLOA 1200 (polyisobutylenesuccinimides), Unithox 750 (ethoxylates), Petronate L (sodiumsulfonate), Disper BYK 101, 2095, 185, 116, 9077 & 220 and ANTI-TERRAseries.

The charge control agent used in all embodiments of the presentinvention is compatible with the solvent in the electrophoretic fluidand may interact with the surface of the charged particles toeffectively generate either positive or negative charge for theparticles.

The term “about” in the present application trefers to a range which is±5% of the indicated value.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particularsituation, materials, compositions, processes, process step or steps, tothe objective, spirit and scope of the present invention. All suchmodifications are intended to be within the scope of the claims appendedhereto.

What is claimed is:
 1. An electrophoretic display comprising anelectrophoretic fluid which comprises charged pigment particles andadditive particles, wherein the additive particles are uncharged orlightly charged neutral buoyancy particles, all types of particles aredispersed in a solvent or solvent mixture, and the concentration of theuncharged or lightly charged neutral buoyancy particles in theelectrophoretic fluid is more than 2.5% by weight, but not exceeding 25%by weight and the additive particles are not seen at the viewing sideduring operation of the display.
 2. The display of claim 1, wherein theuncharged or lightly charged neutral buoyancy particles have the samecolor as one type of charged pigment particles.
 3. The display of claim1, wherein the uncharged or lightly charged neutral buoyancy particleshave a color different from that of the charged pigment particles. 4.The display of claim 1, wherein the uncharged or lightly charged neutralbuoyancy particles are formed from a material selected from the groupconsisting of polyacrylate, polymethacrylate, polystyrene, polyaniline,polypyrrole, polyphenol and polysiloxane.
 5. The display of claim 1,wherein the uncharged or lightly charged neutral buoyancy particles areformed from a material selected from the group consisting ofpoly(pentabromophenyl methacrylate), poly(2-vinylnapthalene),poly(naphthyl methacrylate), poly(alpha-methystyrene), poly(N-benzylmethacrylamide) and poly(benzyl methacrylate).
 6. The display of claim1, wherein the uncharged or lightly charged neutral buoyancy particlesare formed from a material having a refractive index different from thatof the solvent or solvent mixture.
 7. The display of claim 1, whereinthe concentration of the uncharged or lightly charged neutral buoyancyparticles in the electrophoretic fluid is in a range between 3% to 15%by weight.
 8. The display of claim 1 wherein the concentration of theuncharged or lightly charged neutral buoyancy particles in theelectrophoretic fluid is in a range between 3% to 10% by weight.
 9. Anelectrophoretic display comprising an electrophoretic fluid whichcomprises non-transparent charged pigment particles and additiveparticles, wherein the additive particles are colorless transparentparticles, all types of particles are dispersed in a solvent or solventmixture, and the refractive index of the transparent particles issubstantially the same as that of the solvent and the additive particlesare not seen during operation of the display.
 10. The display of claim9, wherein the transparent particles take up less than 20% by volume ofthe fluid.
 11. The display of claim 9, wherein the transparent particlestake up less than 10% by volume of the fluid.
 12. The display of claim9, wherein the transparent particles are formed from a monomer oroligomer selected from the group consisting of acrylate or methacrylate,siloxane modified acrylate or methacrylate, and halogenated acrylate ormethacrylate.
 13. The display of claim 9, wherein the transparentparticles are non-charged.
 14. The display of claim 9, wherein thetransparent particles are charged.
 15. The display of claim 14, whereinthe transparent particles carry a charge the polarity of which is thesame as that carried by one type of the non-transparent charged pigmentparticles, but have a different level of mobility than that of thenon-transparent charged pigment particles.